1. Field of the Invention
The present invention relates to an information recording-reproducing apparatus and a magnetic recording-reproducing head to be mounted thereon, and, more specifically, the present invention relates to an information recording medium that retains information by means of inverted magnetic domains on a magnetic recording film formed on a substrate and an information reproducing apparatus designed to reproduce information by detecting leakage magnetic fluxes from the recording medium.
2. Description of the Background
In recent years, magnetic disk apparatuses have greatly increased in recording density, with the track size for recording bits becoming smaller and smaller. These smaller domains require the magnetic reproducing head to have a higher sensitivity than past devices. One such reproducing head is reported in “Nikkei Electronics” No. 774, Jul. 17, 2000, pp. 177-184. The device disclosed in this reference employs a tunnel magnetoresistive film as a next-generation super-sensitive magnetic sensor.
This first conventional example is characterized by a patterned laminate structure consisting of a lower magnetic shield layer, an electrode, layer, a soft magnetic free layer, a non-magnetic insulating layer, a ferromagnetic pinned layer, an antiferromagnetic layer to fix the direction of magnetization of the ferromagnetic pinned layer, and an electrode layer. The laminate film has, at both ends thereof, a hard magnetic layer to stabilize the direction of magnetization of the non-magnetic free layer and also has an insulating film to insulate the upper and lower magnetic shields.
In the above-mentioned example, the soft magnetic free layer is formed from a CoFe alloy; the non-magnetic insulating film is formed from aluminum oxide; and the ferromagnetic pinned layer is formed from a CoFe alloy. The sensor film has a very low resistance and a very high MR ratio (calculated by dividing the maximum resistance change due to applied magnetic field by the initial resistance) at room temperature. For instance, a sensor film having a resistance per area of 33.5 Ω·μm2 has an MR ratio of 31.6%. A sensor film having a resistance per area of 14.2 Ω·μm2 has an MR ratio of 24.4%. A sensor film having a resistance per area of 5.6 Ω·μm2 has an MR ratio of 12.3%.
A second known example of sensor film is disclosed in Physical Review Letters, Vol. 82, No. 21, pp. 4288-4291. This sensor film employs a laminate film consisting of CoFe alloy, SrTiO3, and La0.7Sr0.3MnO3. It gives a high MR ratio (50% maximum) with a bias voltage (Vs) of −0.4 V at 4.2K.
A third known example of sensor film is disclosed in Physical Review Letters, Vol. 82, No. 3, pp. 616-619. This sensor film employs a laminate film consisting of Ni0.8Fe0.2, Ta2O5, Al2O3, and Ni0.8Fe0.2. It gives an MR ratio of 4% with a bias voltage (Vs) of −0.2 V at room temperature.
The above-mentioned known examples may be characterized by one or more of the following disadvantages. First, with respect to a sensor film having a low resistance and a high MR ratio at room temperature, such a tunnel magnetoresistive sensor in the form of CoFe/Al oxide/CoFe laminate film may include an MR ratio which steeply decreases (as shown in FIG. 11) when a bias voltage is applied across the two CoFe layers. A bias voltage (Vh) of approximately 0.4 V may decrease the MR ratio by up to half from that without bias voltage. When applied to a magnetic read head, this sensor film may have a decreased output in proportion to the bias voltage, unlike the known giant magnetoresistive magnetic read in practical use. Additionally, the tunnel magnetoresistive head, unlike the conventional giant magnetoresistive magnetic read head, may have a decreased signal-to-noise ratio because of its inherent shot noise proportional to the bias voltage.
In order to address one or more of these potential problems and to realize a practical tunnel magnetoresistive head suitable for magnetic recording-reproducing apparatuses with very high recording density, it is preferred to reduce the head resistance. Toward this end, it may be preferable to reduce the thickness of the aluminum oxide film used as the non-magnetic insulating film.
The second known example preferably only needs to meet less stringent requirements for head resistance than the conventional head of the first known example because the MR ratio, which is measured at 5K, reaches the maximum in the vicinity of the head-operating voltage (Vs=−0.5 V). However, a problem may ensure because La0.7Sr0.3MnO3 is a substance which undergoes phase transition from ferromagnetic material to paramagnetic material in the neighborhood of room temperature. In other words, its MR ratio becomes almost zero at 0-60° C. (which is the operating temperature of the magnetic recording apparatus).
In the case of the third known example, the MR ratio reaches a maximum in the vicinity of the head-operating voltage (Vs=−0.2 V). This MR ratio, however, is smaller than that of the giant magnetoresistive head in practical use at the present. Therefore, the third known example may not be suitable for future magnetic recording-reproducing apparatuses with very high recording densities.